The invention relates to a shift mechanism for a motor vehicle gearbox.
In known shift mechanisms, such as those described in German Patent Specification 3,728,903, positive connections between gear wheels and the shafts passing through them are established in order to set up a shifting state. This type of shifting is typical of the shifted transmissions of motor vehicles thereby allowing use of a non-positive synchronizing device, which has the task of matching the speed of the shaft to be coupled to the speed of the gear wheel in engagement.
In a shifting operation, a positive connection is established between a transmission shaft and an idle wheel in engagement. A synchronizing disc is moved along with the idle wheel by a driving cross and, upon meshing of the shift sliding sleeve, establishes a non-positive connection between the latter and the idle wheel. During speed matching, the torque to be transmitted decreases to zero and permits a positive interengaging of shift sliding sleeve and gear wheel.
The positive interengaging is realized by a trapezoidal toothing, the positive profile of which is produced in circle segments laterally on the idle wheel and at the same time is incorporated axially offset in a shift sleeve carrier, which is in firm connection with the shaft. The negative profile for this connection is provided in the inner profile of a shift sliding sleeve and can be pushed over the trapezoidal toothing segments of the idle wheel by axial offsetting.
By this axial offsetting, the positive coupling of the idle wheel with the shift sleeve carrier is brought about. The gripping of the idle wheel by the shift sliding sleeve takes place by means of the coupling toothing which, with the exception of the clearances included for the driving cross described above, has a number of teeth matching the toothed rim of the shift sliding sleeve. These coupling teeth of the toothing segments consequently bring about a positive connection with a statically overdetermined bearing behavior.
Where such shifted transmissions are used in a motor vehicle, frequently a whining gear engaging noise is perceived. This gear whining can be heard distinctly and unpleasantly in the vehicle, causes a considerable impairment in ride comfort. In spite of helical toothing and flank angle correction of the gear wheels used, such cases occur over and over again and in considerable percentages in the series production of shifted transmissions.
The cause is the indeterminate bearing behavior in the coupling toothing, which causes, depending on the variation within the series, an indeterminable pair of teeth for driving and consequently taking over the forces. Since this pair of teeth, determined by the variation within a series, rotates with, the turning of the idle wheel, but the point of force application of the working toothing in the idle wheel remains stationary, there is an interplay between these two forces, which causes a wobbling of the idle wheel. The wobbling of this idle wheel, which is essentially determined in its amplitude by the series-dependent bearing play, causes a cyclical flank angle error in the working toothing, inducing a gear whining noise having an unpleasant effect.
An object of the invention is to avoid the wobbling described here of the idle wheel and the associated disturbing and depreciating noise development in the case of shifted transmissions.
This and other objects are achieved according to an embodiment of the invention in an advantageous way by an idle wheel mounted rotatably and axially undisplaceably on a transmission shaft, on which a coupling ring with an outer axial coupling toothing is arranged coaxially and immovably, a shift sleeve carrier which is arranged immovably on the transmission shaft and has on its outer circumference an outer axial coupling toothing, and an annular shift sliding sleeve, which has on its inner circumference an inner axial coupling toothing corresponding to the outer coupling toothings, in that at least one of the coupling toothings has a tooth pair with two diametrically opposite coupling teeth, each with a pitch deviation leading in the same direction.
An advantage of the invention is that no increased costs are caused in series production. In addition, the invention gives the designer a free hand in designing the shifted transmission with respect to bearing widths of the idle wheel or diameter of the shift sliding sleeve, allowing advantages in reducing the structural form of the gearbox or in eliminating close, and consequently expensive, tolerance limits for the individual components.
The invention consequently relates to the configuration of the trapezoidal coupling toothing of the idle wheel coupling in the shifted transmission. The invention provides a specific configuration of the pitch of this coupling toothing on the idle wheel which ensures that there is always a pair of opposing teeth having the maximum pitch allowance in the pitch diagram and which consequently transform the indeterminate bearing behavior of the driving toothing into a statically determined behavior. A pitch diagram shows the coupling toothing and indicates any deviating from the ideal position in the line on the pitch circle. The coupling toothing expediently has a subdivision into two segments, thereby ensuring a maximum number of pairs of opposite teeth.
In the case of the coupling toothing, this deliberate pitch error according to the invention is produced by the shaping tool which cuts the trapezoidal toothing entering deeper into those tooth gaps which are not intended for assuming the bearing behavior. In the double-segmented coupling toothing, there is further along the trapezoidal toothing a second pair of teeth which can, upon bending of the preceding teeth, then share in taking over the forces after a short bending excursion. In this way it is prevented in particular that, if a torque load is exceeded, a tooth is loaded beyond its bending strength and can thus break off.
The invention has the following effect in the pitch diagram:
In each segment there is a leading pair of teeth, which is produced with phase allowance beyond the distortion to be expected upon hardening, so that it alone can assume force distribution. This pitch configuration can be created in production while the gear wheel is still in the soft state, since although there is still an appreciable distortion of the two segments with respect to each other during hardening, the hardening distortion within a segment may be regarded as minor. The specified amplitude is a value from production experience and does not concern the invention itself. As to the level of pitch lead, it should be noted according to the invention that it must exceed the maximum mispitch of the sliding sleeve involved in the pairing, in spite of hard distortion, in order that the wobble-preventing effect of the teeth with pitch allowance comes to bear in all shift positions.
The other teeth involved in the pitch allowance are not provided with such a great allowance amplitude, but have adequate pitch allowance in order to share proportionately in taking over forces after a small bending excursion without being involved too early in load bearing and consequently disturbing the balancing.
According to an embodiment of the invention, the part of the coupling toothing connected to the gear wheel is provided with a specifically worked pitch pattern, such that in the pitch diagram a pair of opposite teeth have a leading pitch to ensure that the two points of application of the forces for torque transmission are opposite each other, consequently cancel each other out and thus adding no residual force which causes a wobble of the idle wheel.
According to an embodiment of the invention, due to a further pitch pattern, the respectively following teeth are ready to take over the bearing function, consequently, in the event of overloading, a rupture at the root of the first two teeth is prevented by the pitch pattern of the following teeth being kept slightly trailing with respect to the trapezoidal toothing pair intended for bearing, but the following teeth have a lead in the pitch with respect to the rest of the teeth.
According to an embodiment of the invention, the pitch variation for both load directions of a gear wheel is produced in soft working with a basic pitch advance which exceeds the hardening distortion to be expected of the component and consequently ensures effectiveness, irrespective of the variation within the series, but which at the same time is only of such a size that the interval between thrust and pull in the trapezoidal toothing has adequate play in order to ensure smooth shifting of the gearbox and at the same time that the impact upon load changing does not increase unpleasantly after intensive wear.
According to an embodiment of the invention, for wobble-free coupling of the type described here, an even-numbered segment division is to be chosen for the design of the trapezoidal toothing on the idle wheel in order that the trapezoidal toothing has a greater number of opposite teeth.
According to an embodiment of the invention, to avoid wobbling of the subassembly comprising shift sliding sleeve and idle wheel, the same pitch variation is also produced on the coupling toothing of the shift sleeve carrier, which generally does not have any segmenting but has all the teeth incorporated in the outer profile, in such a way that here likewise two teeth are intended for bearing by leading pitch deviation and a number of following teeth are kept ready to take over the force, the basic pitch advance being adapted to correspond to the hardening distortion to be expected in the series production of this component.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.